Affiliation

Abstract

Permanent hearing loss is caused by the irreversible damage of cochlear sensory hair cells and nonsensory supporting cells. In the postnatal cochlea, the sensory epithelium is terminally differentiated, whereas tympanic border cells (TBCs) beneath the sensory epithelium are proliferative. The functions of TBCs are poorly characterized. Using an Axin2(lacZ) Wnt reporter mouse, we found transient but robust Wnt signaling and proliferation in TBCs during the first 3 postnatal weeks, when the number of TBCs decreases. In vivo lineage tracing shows that a subset of hair cells and supporting cells is derived postnatally from Axin2-expressing TBCs. In cochlear explants, Wnt agonists stimulated the proliferation of TBCs, whereas Wnt inhibitors suppressed it. In addition, purified Axin2(lacZ) cells were clonogenic and self-renewing in culture in a Wnt-dependent manner, and were able to differentiate into hair cell-like and supporting cell-like cells. Taken together, our data indicate that Axin2-positive TBCs are Wnt responsive and can act as precursors to sensory epithelial cells in the postnatal cochlea.

Figures

Fig. 1.

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Axin2 expression in tympanic border…

Fig. 1.

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Axin2 expression in tympanic border cells and their lineage origin. ( A )…

Proliferation and Wnt signaling decreased with age in the postnatal cochlea. ( A-C…

Fig. 2.

Proliferation and Wnt signaling decreased with age in the postnatal cochlea. (A-C) Cryosections of cochleae from mice administered EdU once daily for 2 days. EdU-labeled TBCs decreased from P3 to P9, and further decreased from P9 to P23. (D) Quantification of EdU-positive TBCs (n=3-10). (E) The number of TBCs (between the lateral cochlear wall and habenula perforata) decreases with age, n=6 for each age. (F) Using RT-PCR and qPCR, cochleae demonstrated a significant decrease in the Wnt target genes (Axin2 and Sp5) (P<0.005 for both) between P2 and P15; experiments were carried out in triplicate. Levels of the hair cell marker (Brn3.1) remained constant (P=0.24). Levels of the supporting cell marker (Sox2) decreased (P<0.05) and those of the outer hair cell gene Prestin increased (P<0.001). (G-J) Axin2lacZ expression in the postnatal cochlea decreased with age. Scale bars: 25 μm in A-C; 200 μm in G-J. Data are mean±s.d. Asterisk indicates statistical significance.